1. Field of the Invention
This invention relates to a process and apparatus for polymerizing fluoroolefins in the gas phase.
2. References
Polymerization of tetrafluoroethylene (TFE) in the gas phase in the presence of nonvolatile initiators is known. For example, U.S. No. 3,592,802 discloses gaseous polymerization of TFE at about 40.degree. to 120.degree. C. in the presence of selected di(saturated hydrocarbyl) peroxydicarbonates such as diisopropylperoxydicarbonate, preferably supported on a suitable carrier material such as polytetrafluoroethylene. However, product adhered to the cooling coils. U.S. No. 3,304,293 discloses gas phase polymerization of TFE employing heat-activated gels of silica or silica-alumina admixed with salts of oxy acids of hexavalent chromium, such as magnesium chromate. U.S. No. 3,170,858 discloses gaseous polymerization of TFE, or copolymerization thereof with minor amounts of other fluoroolefins such as hexafluoropropene or vinylidene fluoride, in the presence of previously irradiated TFE homopolymer.
Use of nonvolatile initiators, including fluorinated oligomers, in condensed phase polymerization of TFE is also known. For example, U.S. No. 3,493,530 discloses polymerization of perfluorinated olefins in the presence of macromolecular perfluorinated polyperoxides of the formula (C.sub.3 F.sub.6 O.sub.x).sub.n wherein n is an integer of 5 to 100, the initiators being soluble in the liquid polymerization media. The use of TFE/ether- or HFP/ether-peroxide copolymers which are nonvolatile oils as initiators for fluorolefin polymerization is disclosed in Dutch Application No. 6,711,121. U.S. No. 2,598,283 discloses copolymerization of TFE and HFP employing bis-trichloroacetyl peroxide at low temperatures in a perhalogenated hydrocarbon solvent. U.S. Nos. 4,535,136 and 4,588,796 (incorporated herein by reference) disclose solution polymerization of fluoroolefins employing acyl hypofluorite initiators of the formula RCOOF or X(CF.sub.2).sub.n COOF wherein R is X(CF2).sub.n --or CF.sub.3 CF.sub.2 CF.sub.2 O[CF(CF.sub.3)CF.sub.2 O].sub.m CF(CF.sub.3)--, X is H or F, m is 0 to 50 and n is 1 to 16.
U.S. No. 2,753,329 discloses gaseous polymerization of TFE employing peroxide and peracetate catalysts which reportedly produce a fluidizable, powdery polymer which does not adhere to the reactor walls. Polymerization is carried out at a temperature of about 125.degree. to about 200.degree. C. and a pressure of at least 100 psi (690 kPa). Under these process conditions, more than 99% of the catalyst is said to be present in the vapor phase.
Prior art processes for polymerizing fluoroolefins in the gas phase sometimes employ inert gases or vapors as heat transfer media to remove heat of polymerization. Although recycle of monomers has not been reported for cooling fluoroolefin polymerization, recycle has been employed with non-halogenated monomers. U.S. No. 4,525,547 discloses recycle of unreacted monomers through external heat exchangers to remove heat of polymerization in the gas phase polymerization of ethylene. Copolymerization of ethylene with other alpha-olefins employing a non-volatile (liquid or solid) coordination catalyst, an inert gaseous hydrocarbon diluent, and recycle of unreacted monomers through an external heat-exchanger is disclosed. The process reportedly results in, among other things, "reduced polymer deposition on the inner wall of the polymerization vessel, the prevention of lumpy polymer formation, and the proceeding of uniform copolymerization reaction".
Typical gas phase fluoroolefin polymerization processes of the art result in reactor fouling and plugging requiring frequent and costly shutdowns. Copending U.S. Ser. No. 07/166484, filed Mar. 10, 1988, now U.S. Pat. No. 4,861,845, solves these reactor fouling and plugging problems by utilizing a recycle gas stream heat exchanger to cool unreacted monomer gas. This process adds a non-volatile halogenated free radical source (initiator) directly to the polymerizer. Because the initiator is non-volatile, it is not taken up into the recycle gas stream. While this process is effective, it requires that polymerization occur at a temperature sufficiently high to decompose the initiator selected, which may be too high for avoiding rearrangement of some comonomers (e.g., perfluoro [alkyl vinyl]ethers), leading to unstable end groups in the polymer formed. It also requires use of a non-volatile initiator which is soluble in a non-telogenic solvent so it can be fed to the polymerizer.